1. Field of the Invention
The present invention relates to an electronic balance of the electromagnetic force balancing type and, more particularly, to the control of a beam of an electronic balance and to the measurement of a weight value.
2. Description of the Related Art
In an electronic balance of the electromagnetic force balancing type, a coil is attached to a moving part of a beam that is displaced according to a weight to be measured. This coil is placed in a magnetic field generated by a permanent magnet. An electromagnetic force, which is generated by passing electric current through the coil, and the weight to be measured are balanced. Thus, the magnitude of the weight to be measured is determined according to the coil current, which is obtained when the weight to be measured and the electromagnetic force are balanced, or to a control signal used for determining such a coil current.
The coil current is determined according to the amount of displacement of the beam, which is detected by a displacement detector for detecting the displacement of the moving part of the beam, and feedback-controlled so that the moving part of the beam is displaced in such a way as to maintain a balanced point (or zero point).
Usually, PID (Proportional, Integral, and Differential) control is performed as the feedback control applied to the coil current. According to the PID control of the electronic balance, the value of a controlled variable is determined by the following equation and outputted as an amount of the coil current.
[Equation 1]Y=P×X+I×∫Xdt+D×dX/dt  (1)where “Y” designates an amount of the coil current, “X” denotes an amount of displacement of the beam, “P” designates a proportional constant, “I” denotes an integral constant, and “D” designates a differential constant.
That is, a synthetic PID control signal is synthesized from (or by adding up) a component being proportional to the amount X of displacement of the beam, an integral component of the amount X thereof, and a differential component of the amount X thereof, and outputted as the amount Y of the coil current.
Conventional PID control of an electronic balance employs an analog method of determining a P control signal, an I control signal and a D control signal as signals, which represent analog amounts, by performing analog computations according to the amount of displacement of the beam, which is detected by the displacement detector as an analog amount and by then generating a coil current according to the control signals. However, as a digital control technique advances, there has become widespread an electronic balance employing a digital PID control method of determining a P control signal, an I control signal and a D control signal as signals, which represent digital amounts, by A/D-converting (or digital-converting) the amount of displacement of the beam, which is detected by the displacement detector as an analog amount, and by then performing digital computations on the P control signal, the I control signal, and the D control signal and of subsequently D/A-converting (or analog-converting) the P control signal, the I control signal, and the D control signal, which represent digital amounts, to thereby produce a coil current (see, for instance,) JP-A-10-19642).
The digital method has an advantage in that a user can easily change the set values of the proportional constant (P), the integral constant (I) and the differential constant (D) in the equation (1) in a case where the set values of the proportional constant (P), the integral constant (I) and the differential constant (D) are changed according to a user environment (for example, in a case where the constants are changed in the user environment, in which microvibrations frequently occur, so as to suppress the influence of the microvibrations. That is, alteration of the set values of the constant in the analog method requires the adjustment of a semi-fixed resistor or the like included in the analog computation circuit and compels the user himself to perform a difficult operation of adjusting the semi-fixed resistor or the like. In contrast, although the digital method needs an input device, such as a key switch, for inputting a set value into the digital computation circuit, the user has only to input an appropriate constant value thereto by using the input device. Thus, the user can easily change the set values of the constants.
However, the analog PID control method and the digital PID control method have advantages and drawbacks other than those described hereinabove.
The analog PID control method has advantages in that a processing time for the A/D conversion, the computation and the D/A conversion is unnecessary, and that a control operation can be performed without time delay in responding to displacement of the beam.
Meanwhile, the analog method needs to digitize the analog amount when the weight value is indicated as a digital value (even in the case of employing the analog feedback control, it is general to indicate a weight value as a digital value). At that time, an error (a quantization error) due to the A/D conversion is caused between a weight value (an analog amount), which corresponds to the feedback coil current, and a weight value (a digital value) indicated in an indicator as a result of measurement. That is, the coil current needed for the feedback control is determined according to the analog amount of which an A/D conversion is not performed, whereas the weight value indicated in the indicator is determined according to the digital value obtained after the A/D conversion. Thus, the quantization error is caused therebetween.
Meanwhile, the digital PID control method determines both the coil current value and the weight value according to the same digital amount obtained after the A/D conversion (because an amount obtained by D/A-converting the same digital value as that used for determining the indicated weight value is used as the analog amount used for determining the coil current (an analog amount), no quantization error is caused between the weight value corresponding to the coil current and the indicated weight value). Thus, the digital method has an advantage in that no error due to digitization is caused. However, according to the digital method, a control operation performed in response to displacement of the beam is delayed, as compared with that performed according to the analog method. Thus, control stability is degraded.
Thus, the analog control method and the digital control method have contradictory characteristics relating to feedback control stability and measurement accuracy. Further, the digital control method features that the set values of the PID constants can easily be changed by annexing an input apparatus to the electronic balance. Consequently, the recent electronic balance employs one of the analog control method and the digital control method according to the purpose of use.
Not only the capability to achieve an accurate measurement with a small measurement error (including quantization error) but the capability to enable a stable measurement with excellent measurement responsivity are simultaneously required of the electronic balance.
As compared with the aforementioned two methods, the analog control method is superior in responsivity associated with the displacement and excels in stability. However, the analog method is inferior in measurement resolution and measurement accuracy. Conversely, the digital control method is superior in the measurement resolution and the measurement accuracy and inferior in the stability. Thus, according to the conventional analog control method and to the conventional digital control method, it is difficult to provide an electronic balance enabled to perform measurement with high resolution and accuracy and with high stability.